1. Field of the Invention
This invention relates to low loss surface acoustic wave (SAW) filters and to a method of operation thereof. More particularly, this invention relates to the realization of low loss resonator type SAW filters with good rejection performance using non-sequential coupling, often referred to as cross-coupling.
2. Description of the Prior Art
SAW filters are known and can achieve good-band loss performance but the roll-off (rejection) characteristics are generally unsatisfactory. Known low-loss SAW devices that utilize resonator technology suffer from unacceptable rejection characteristics. Good rejection characteristics can be achieved using transversal SAW filter technology, but the loss of these filters increases. Although some progress has been made with unidirectional transversal filters that exhibit both "low" loss and good rejection, the loss of these devices is still much higher than that achievable using SAW resonator filters. Also, unidirectional transversal filters are relatively expensive compared to SAW resonator filters in relation to design costs, manufacturing costs, external matching hardware required and size.
Resonator filters are generally used in Personal Communication Systems (PCS) and low power applications. Transversal filters (with amplification to compensate for the high loss) are used in the more demanding communications satellite market.